The present invention relates to a method for sulfur protection of a NOx adsorber, and especially relates to the use of fuel to protect a NOx adsorber against sulfur poisoning.
It is well known in the art to use catalyst compositions, including those commonly referred to as three-way conversion catalysts (xe2x80x9cTWC catalystsxe2x80x9d) to treat the exhaust gases of internal combustion engines. Such catalysts, containing precious metals like platinum, palladium, and rhodium, have been found both to successfully promote the oxidation of unburned hydrocarbons (HC) and carbon monoxide (CO) and to promote the reduction of nitrogen oxides (NOx) in exhaust gas, provided that the engine is operated around balanced stoichiometry for combustion (xe2x80x9ccombustion stoichiometryxe2x80x9d; i.e. between about 14.7 and 14.4 air/fuel (A/F) ratio).
Fuel economy and global carbon dioxide (CO2) emissions have made it desirable to operate the engine under lean-burn conditions, where the air-to-fuel ratio is somewhat greater than combustion stoichiometry (i.e., greater than 14.7 and generally between 19 and 35), to realize a benefit in fuel economy. When lean-burn conditions are employed, three way catalysts are efficient in oxidizing the unburned hydrocarbons and carbon monoxides, but are inefficient in the reduction of nitrogen oxides.
One approach for treating nitrogen oxides in exhaust gases of engines operating under lean-burn conditions has been to incorporate NOx adsorbers in exhaust lines along with three way catalysts. Conventional exhaust systems contemplate various configurations, including for example, use of NOx adsorbers in the same canister along with three-way catalysts or use of a NOx adsorber in a separate can upstream of the three-way catalyst, among others.
These adsorbers generally comprise one or more catalytic metal(s), such as platinum, palladium and/or rhodium, in combination with an alkali and/or alkaline earth element, loaded on a porous support such as alumina, gamma-alumina, zirconia, titania, alpha-alumina, cerium oxide (ceria), lanthanum oxide, or magnesium oxide. The catalytic material in the adsorber acts first to oxidize NO to NO2. NO2 then reacts with the alkali and alkaline earth element to form stable nitrate salts. In a stoichiometric or rich environment, the nitrate is thermodynamically unstable. Consequently, the stored NOx is released for catalysis, whereupon NOx is reduced to N2 gas.
For practical incorporation of the supported catalytic materials into internal combustion engine exhaust systems, the support will, itself, be deposited on a chemically stable and thermally insulating substrate. Particularly useful substrates include cordierite and mullite, among others. The substrate may be of any size or shape, such as is required by the physical dimensions of the designed exhaust system. Similarly, the internal configuration of the substrate may be any known or commonly employed configuration. Substrates are typically formed as monolithic honeycomb structures, layered materials, or spun fibers, among other configurations.
U.S. Pat. No. 5,727,385 to Hepburn, which is herein incorporated by reference, discloses a NOx trap, comprising (i) at least one precious metal selected from platinum and palladium loaded on a porous support; and (ii) at least one alkali or alkaline earth metal (a) loaded on a porous support or (b) present as an oxide thereof. Hepburn optionally includes a three-way catalyst located either between the two components or after the NOx trap.
Although the NOx adsorbers remove the NOx from the exhaust stream during lean burn conditions and/or low temperatures, they are plagued with the problem of sulfur poisoning under such conditions. Sulfur, a contaminant present in fuel, adsorbs onto the NOx adsorber, reducing the sites available for trapping NOx.
What is needed in the art is an exhaust gas catalyst system having improved durability, as well as NOx and sulfur management, over extended operating time.
The above-described and other disadvantages of the prior art are overcome by the exhaust gas systems of the present invention and the methods for regenerating a particulate trap and for regenerating a sulfur trap. One embodiment of the system comprises a sulfur trap disposed within an exhaust stream, the sulfur trap comprising a sulfur scavenger component, a NOx adsorber disposed within the exhaust stream, downstream from the sulfur trap, and a fuel introduction point located between the NOx adsorber and the sulfur trap.
Another embodiment of the system comprises a particulate trap disposed within an exhaust stream, a NOx adsorber disposed within the exhaust stream, downstream from the particulate trap, and a fuel introduction point located between the NOx adsorber and the particulate trap.
The method for regenerating a sulfur trap in an exhaust system having the sulfur trap disposed upstream of a NOx adsorber, comprises introducing at least a portion of a fuel rich exhaust stream to the sulfur trap and removing sulfur species from the sulfur trap. Then, before the exhaust stream is directed from the sulfur trap through the NOx adsorber, fuel is introduced to the NOx adsorber.
The method for regenerating a particulate trap in an exhaust system having a particulate trap disposed upstream of a NOx adsorber, comprises introducing at least a portion of the exhaust stream to the particulate trap and regenerating the particulate trap to produce a hot exhaust stream. Then, prior to directing the hot exhaust stream through the NOx adsorber, fuel is introduced to the NOx adsorber.
The above-described and other features and advantages of the present invention will be appreciated and understood by those skilled in the art from the following detailed description, drawings, and appended claims.